Many techniques for the generation of glow discharge plasmas for the purpose of depositing thin film materials have been investigated. These techniques have included methods of plasma generation through the use of direct current and alternating current at sub-atmospheric pressure conditions. Various AC frequencies have been utilized, such audio, radio frequency (R.F.) and a microwave frequency of about 2.56 GHz. The assignee of the instant invention, has disclosed, in U.S. Pat. No. 4,504,518, entitled METHOD OF MAKING AMORPHOUS SEMICONDUCTOR ALLOYS AND DEVICES USING MICROWAVE ENERGY, the disclosure of which is incorporated herein by reference, that the optimum combination of power and pressure at which to deposit thin film material by plasma enhanced chemical vapor deposition processes is defined by the substantial minimum of the modified Paschen curve for the particular precursor gaseous mixture being used. The modified Paschen curve defines the voltage needed to sustain the glow discharge plasma at each pressure in a range of pressures.
In a typically sized, conventional R.F. glow discharge system, the minimum of the modified Paschen curve was found to occur at a few hundred millitorr. By increasing the applied R.F. power, the gaseous utilization efficiency and the deposition rate also increased. However, simply increasing the R.F. power to increase deposition rates, also leads to the production of films of decreasing quality and can additionally result in the (1) generation of powder and (2) deposition of films which include polymeric material. While the increased deposition rate with increased R.F. power is a result of an increase in the concentration of excited species resulting principally from collisions between electrons and feedstock molecules; the collision rate between excited species (and more importantly between excited species and feedstock molecules) is also increased. This results in the formation of polymeric chains. In order to reduce the number of undesirable collisions, it is possible to reduce the operating pressure, but such a reduction in pressure moves the deposition process up from the minimum of the modified Paschen curve; the result being that substantially greater R.F. power is required in order to achieve the same degree of plasma excitation. This is because, as the operating pressure is reduced, many electrons that would have collided with and excited feedstock molecules at higher operating pressures, now impinge on either the substrate or system walls without suffering any inter-molecular collisions.
In the aforementioned +518 patent, the assignee of the instant invention discloses that for a given deposition system, the minimum in the modified Paschen curve shifts to lower pressure values with increasing frequency, such as microwave frequencies, vis-a-vis, R.F. frequencies. Therefore, the use of high frequency microwave energy in a glow discharge deposition system was found to provide for operation of that system at greatly reduced pressure. The result was the ability to generate a higher concentration of excited species, and thus to provide higher rates of deposition and gas utilization efficiencies without the production of powder or the inclusion of polymeric species in the depositing film. The shift in the minimum of the modified Paschen curve to lower pressures as the frequencies increase is believed to occur because, for a given gas pressure at the higher excitation frequencies, the rapid reversals of the applied electric field allows electrons to collide with more feedstock molecules in the plasma excitation region before encountering the walls of the system.
While the assignee of the instant invention has previously discovered the improved efficiency in manufacturing economy as well as the improved material quality capabilities which can be provided by (1) operating at the substantial minimum of the modified Paschen curve and (2) utilizing microwave enhanced chemical vapor deposition, the aforementioned '518 patent had a significant drawback. Said patent required that the microwave energy be introduced into the interior of a vacuumized deposition chamber from a source maintained at atmospheric pressure. This meant that (1) complex vacuum sealing of the microwave waveguide (or other microwave transmitting apparatus) to the vacuum chamber had to be effected and (2) the coupling of that microwave energy into the interior of the vacuum chamber had to be effected. Further, all such prior art microwave enhanced, chemical vapor deposition processes of which the instant inventors are aware employ a relatively large vacuum chamber. The utilization of large vacuum chambers has the inherent disadvantage of requiring correspondingly large gas throughput and vacuum pumping capabilities regardless of the size of the substrate. It is, therefore, one major objective of the instant invention to substantially eliminate all of the hereinabove enumerated deficiencies of prior art systems.